1. Technical Field
This invention relates to an electromechanical linear-motion actuator for converting the rotation of a rotary shaft driven by an electric motor to axial movement of planetary rollers, and an electromechanical brake system in which this electromechanical linear-motion actuator is used.
2. Background Art
Conventional vehicle brake systems are mostly hydraulic systems, of which brake pads are driven by hydraulic cylinders and pressed against a brake disc. But with an introduction of anti-lock brake systems (ABS) and other brake control arrangements, electromechanical brake systems, which use no hydraulic circuit, are gathering attention these days. An electromechanical brake system includes an electromechanical linear-motion actuator mounted in a caliper body to convert the rotation of a rotary shaft driven by an electric motor to axial movement of planetary rollers. The axial movement of the planetary rollers is transmitted to brake pads to press the brake pads against a brake rotor, thereby producing braking force.
FIG. 25 shows such an electromechanical linear-motion actuator, which comprises an electric motor 90 having a rotary shaft 91, a stationary outer race member 92 provided around the rotary shaft 91, a plurality of planetary rollers 93 disposed between the radially outer surface of the rotary shaft 91 and the radially inner surface of the outer race member 92 so as to be rotated about their own axes while revolving about the rotary shaft 91 when the rotary shaft 91 rotates. A helical rib 94 is provided on the radially inner surface of the outer race member 92, and a helical groove 95 is formed in a radially outer surface of each of the planetary rollers 93 in which the helical rib 94 is engaged. The actuator further includes a planetary carrier 96 axially facing the planetary rollers 93, thrust ball bearings 97 disposed between the planetary carrier 96 and the respective planetary rollers 93 for preventing the rotation of the respective planetary rollers 93 from being transmitted to the planetary carrier 96, a moving member 98 axially facing the planetary carrier 96, and a thrust rolling bearing 99 disposed between the moving member 98 and the planetary carrier 96 for preventing the revolving motion of the planetary carrier 96 from being transmitted to the moving member 98 (JP Patent Publication 2007-37305A).
In this electromechanical linear-motion actuator, when the rotary shaft 91 rotates, the planetary rollers 93 rotate about their own axes while revolving around the rotary shaft 91. Simultaneously, the planetary rollers 93 are moved axially with their helical grooves 95 guided by the helical rib 94 of the outer race member 92. The axial movement of the planetary rollers 93 is transmitted through the thrust ball bearings 97 and then the planetary carrier 96 to the moving member 98.
It is desired that such an electromechanical linear-motion actuator is as short in axial length as possible. The shorter the axial length of this actuator, the more it is possible to reduce the axial length of the electromechanical brake system. This in turn makes it possible to increase the freedom of layout of component parts around the electromechanical brake system (such as a suspension). But actually, the above-mentioned conventional electromechanical linear-motion actuator has a rather long axial length.